Use RequestURI when redirecting to canonical path. (#1331)

* Use RequestURI when redirecting to canonical path.

Caddy may trim a request's URL path when it starts with the path that's
associated with the virtual host. This change uses the path from the request's
RequestURI when performing a redirect.

Fix issue #1327.

* Rename redirurl to redirURL.

* Redirect to the full URL.

The scheme and host from the virtual host's site configuration is used
in order to redirect to the full URL.

* Add comment and remove redundant check.

* Store the original URL path in request context.

By storing the original URL path as a value in the request context,
middlewares can access both it and the sanitized path. The default
default FileServer handler will use the original URL on redirects.

* Replace contextKey type with CtxKey.

In addition to moving the CtxKey definition to the caddy package, this
change updates the CtxKey references in the httpserver, fastcgi, and
basicauth packages.

* httpserver: Fix reference to CtxKey

caddyhttp/httpserver/replacer.go

@@ -13,6 +13,8 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"github.com/mholt/caddy"
 )
 
 // requestReplacer is a strings.Replacer which is used to
@@ -299,7 +301,7 @@ func (r *replacer) getSubstitution(key string) string {
 		}
 		return requestReplacer.Replace(r.requestBody.String())
 	case "{mitm}":
-		if val, ok := r.request.Context().Value(CtxKey("mitm")).(bool); ok {
+		if val, ok := r.request.Context().Value(caddy.CtxKey("mitm")).(bool); ok {
 			if val {
 				return "likely"
 			} else {